Method for calibrating a pair of body-worn sensors - Patent Application 20070122997

Abstract

1. A method of calibrating a pair of body-mounted sensors, the method comprising: (a) determining, at a baseline position of the joint to be measured, a first offset between a measured joint angle and the angle between a pair of sensors, one attached to each side of the joint to be measured, thereby calibrating the sensors; (b) setting the joint back to the baseline position after at least one of the sensors has been removed and reattached, whereby the sensors are in a second configuration relative to each other; and (c) determining a second offset between the measured knee angle and the angle between the pair of sensors in the second configuration, to recalibrate the sensors, whereby the first configuration and the second configuration each report the same joint angle relative to the baseline position.

Description

[Technical Field]

[0001] [CROSS-REFERENCE TO RELATED APPLICATIONS] This application claims the benefit of priority from UK Patent Application No. 1915135.6 filed on 18 October 2019, the disclosure of which is incorporated herein by reference.

[0002] The present invention relates to a method for calibrating sensors to compensate for misalignment, typically of the sensor relative to the body, and a system for attaching sensors to the body to reduce misalignment. [Background technology]

[0003] Devices that measure motion are becoming increasingly popular. These sensing devices can take the form of wearable devices that measure a user's motion, smartphones held by a user to measure the user's motion, or movable devices that can generally sense motion, such as sensors attached to video game controllers or industrial equipment. Specifically, wearable devices can be utilized to track the motion of humans or other animals. Specifically, they can be used to monitor the motion of specific joints.

[0004] These mobile sensing devices may include a satellite positioning sensor capable of detecting the location of the device, and one or more motion sensors that detect the motion and / or orientation of the device (i.e., motion and / or orientation). These motion sensors may include one or more of an accelerometer, a gyroscope, a magnetometer, a compass, and a barometer.

[0005] When using a wearable device, it may be necessary to use the device for an extended period of time, such as a month or more, so as to accumulate data that changes only slowly over time. This means that any sensing device used may need to be removed for any number of reasons, including, but not limited to, the need to recharge the power supply on the device, the accumulation of dirt on the sensor and the desire to clean the sensor to remove dust or spills, or to wash the part of the person or animal to which the sensor is attached.

[0006] While simple methods of attaching a wearable device may include one or more ties, straps, or belts, or other attachment systems that allow for simple and easy removal, such devices may be uncomfortable for the user to whom the device is attached.

[0007] Additionally, removing a sensor from a user and then replacing or reinstalling the sensor provides a significant opportunity to replace the sensor in a different location relative to the previous sensor installation. This can also lead to problems and inconsistencies in the recorded data, potentially rendering some or all of the data unusable. This is especially true when two or more sensors work together to provide data regarding the relative movement of the sensors. Summary of the Invention [Problem to be solved by the invention]

[0008] Therefore, it is desirable to have improvements in how wearable sensors can be attached and operated. [Means for solving the problem]

[0009] According to the present invention, there is provided a method of calibrating a pair of body mounted sensors, the method comprising: (a) determining a first offset between the angle between the pair of sensors, one mounted on each side of the joint to be measured, at a baseline position of the joint to be measured and the measured joint angle, to thereby calibrate the sensors; (b) setting the joint to return to the baseline position after at least one of the sensors has been detached and reattached, whereby the sensors are in a second configuration relative to one another; and (c) determining a second offset between the measured knee angle and the angle between the pair of sensors in the second configuration, to recalibrate the sensors, whereby the first configuration and the second configuration each report the same joint angle relative to the baseline position.

[0010] A pair of sensors can communicate such that the angle between the sensors is determined by one of the sensors.

[0011] The method may further include, before (a), measuring the joint angle by using a goniometer.

[0012] Recalibration may be performed as part of the sensor start-up process.

[0013] The step of measuring the baseline position may include measuring the joint angle between each part of the joint, for which a goniometer may be used. The measured joint angle may be a pitch angle and / or a roll angle.

[0014] The method may further comprise the step of moving the joint to a baseline position, preferably in a fully extended position of the joint.

[0015] Preferably, reattaching a removed sensor is performed in substantially the same position as it was previously set.

[0016] The method may further include identifying an axis of motion of the joint.

[0017] The method may further include applying the sensors, one on each side of the joint. The method may further include marking sensor locations on each side of the joint before applying the sensors.

[0018] The present invention also provides a system for recording changes in angular position of a joint, comprising a pair of sensors, each sensor being positioned, in use, on a respective side of the joint, and each sensor including a data transmitting device for providing data relating to an orientation of the sensor; a data storage device for receiving data from one or more of the sensors, the data relating to the orientation of one or both sensors; and a control system configured to recognise when a sensor has been removed from the joint and to require recalibration of the sensor alignment prior to recording a subsequent data set.

[0019] The present invention further provides a system for attaching a removable sensor to a body of an animal for a period of time, the system comprising: a first mount having an adhesive layer on one surface for attachment to a surface of the animal's body for a first subset of the period of time; and a second mount operative to removably secure the sensor to the first mount for a second subset of the period of time, the second subset being shorter than the first subset of the period of time.

[0020] The second mount may be two-way secure.

[0021] The second mount is (i) a first temporary fixation system for enabling a second mount to be fixed to the first mount for a second subset of the time period; (ii) a second temporary fixation system for enabling the sensor to be fixed to a second mount; and It can be equipped with:

[0022] A plurality of second mounts can be provided, typically a plurality of second mounts sufficient to allow repeated attachment of sensors to the first mounts within the first subset of time periods.

[0023] A plurality of first mounts may be provided to allow replacement of the first mount after a first subset of periods of time.

[0024] The second mount may include one or more of an adhesive, a hard clip, a soft pocket, a press fit fitting, a directional hook and loop fastener (Velcro®), or a magnet.

[0025] The first mount can include at least one visual indicator section through which respective marks on the animal's body can be viewed to assist in alignment of the replacement first mount. Two or more visual indicator sections may be provided.

[0026] The first mount may comprise a multi-layer structure, preferably having layers comprising MED 2171 H, polyurethane film, and MED 5062 A.

[0027] The second mount can have adhesive on both sides. The second mount may include a layer formed of MED 6361U.

[0028] The second mount can be in two parts, the first part for attachment to the first mount and the second part for attachment to the sensor, whereby the fixation couples the first and second parts together.

[0029] The present invention also provides a method as described in accordance with any combination of the above features, wherein one or more of the sensors are attached to the body using a system as described in accordance with any combination of the above features.

[0030] The invention will now be described, by way of example only, with reference to the accompanying drawings, in which: [Brief explanation of the drawings]

[0031] [Figure 1] FIG. 1 is a diagram illustrating a joint. [Figure 2] FIG. 1 illustrates a directional reference frame for a joint. [Figure 3] FIG. 1 illustrates a pair of sensors fitted on either side of a joint, according to some embodiments. [Figure 4A] FIG. 4(a) is a diagram showing a sensor mounting system. [Figure 4B] FIG. 4(b) is a diagram showing a sensor mounting system. [Figure 4C] FIG. 4(c) shows a sensor mounting system. [Figure 5] FIG. 1 shows a schematic method for operating and / or calibrating a sensor. [Figure 6A] FIG. 6(a) is a diagram showing a method for calibrating a sensor. [Figure 6B] FIG. 6(b) is a diagram showing a method for calibrating the sensor. [Figure 6C] FIG. 6(c) is a diagram showing a method for calibrating the sensor. [Figure 6D] FIG. 6(d) shows a method for calibrating the sensor. [Figure 6E] FIG. 6(e) is a diagram showing a method for calibrating the sensor. [Figure 7A] FIG. 7(a) is a diagram illustrating a method of using the sensor mounting system. [Figure 7B] FIG. 7(b) illustrates a method of using the sensor mounting system. [Figure 7C] FIG. 7(c) illustrates a method of using the sensor mounting system. [Figure 7D] FIG. 7(d) illustrates a method of using the sensor mounting system. [Figure 7E] FIG. 7(e) is a diagram illustrating a method of using the sensor mounting system. DETAILED DESCRIPTION OF THE INVENTION

[0032] Although this specification describes a specific example of the use of sensors in relation to a human knee joint, the underlying principles are applicable to many different joints, such as the hip, shoulder, ankle, elbow, or wrist, and may also be applied to joints associated with other animals.

[0033] FIGS. 1 and 2 are provided to allow for a simple explanation of certain terms used herein. FIG. 1 illustrates a standard leg having a femur 1, a tibia 2, and a fibula 3, which are joined at a knee joint 4. The femur defines a mechanical axis 5 of the thigh that extends from the knee to a ball joint 6 that forms part of the human hip. The mechanical axis 7 of the tibia extends from the knee 4 to the lower end 8 of the tibia itself. The femur and shin (formed by the tibia 2 and fibula 3) can pivot relative to one another about the knee joint axis 9. As such, the femur and shin define a plane in which their respective mechanical axes pivot relative to one another. Each mechanical axis will thus be substantially aligned with its respective part of the leg, such that the knee joint axis 9 is perpendicular to the plane in which the axes pivot. The knee angle is thus typically the angle between the two mechanical axes. This is an idealized situation that forms the basic geometry considered by the present invention. To address any misalignment between the axis and the respective parts of the leg, one or more correction schemes can be applied.

[0034] Figure 2 helps define the coordinate system associated with the knee joint and how the terms pitch and roll apply to the knee. The convention when discussing the knee joint is that when a person is standing upright, the x-axis points forward parallel to the ground, i.e., away from the knee; the y-axis points toward the person's right; and the z-axis points downward toward the ground. This convention applies to both the left and right legs; i.e., the positive side of the y-axis is always to the right of the knee, regardless of the leg. Thus, in normal knee alignment, the y-axis is analogous to the knee joint axis 9.

[0035] The orientation of any sensor relative to the knee typically has two components: rotation of the sensor about the x-axis is the roll motion, identified by arrow 18, and defines the roll angle; rotation of the sensor about the y-axis is the pitch motion, identified by arrow 19, and defines the pitch angle.

[0036] FIG. 3 shows a pair of sensors 10 affixed to a leg 11. Each sensor contains one or more motion-sensing devices that enable either (i) determining the pitch and / or roll and / or yaw of the individual sensor or (ii) determining the relative pitch and / or roll and / or yaw between the sensors. The motion-sensing devices can be any suitable device, such as, but not limited to, an accelerometer, a gyroscope, or a pair of strain gauges. In other variations, more than two sensors can be used. For example, if the joint under monitoring is a ball-and-socket joint with three degrees of freedom of motion, it may be desirable to use three sensors. In some cases, it may be desirable to use additional sensors at a joint, such as the knee, to help determine the orientation of the thigh and calf. For example, two sensors can be placed on one of the user's limbs. Measurements from such a third sensor can be processed in a manner similar to that described above for two sensors. One possibility is to process data from two sensors placed on one limb to obtain a set of data for that limb, which is then processed in conjunction with data from the other limb as described above.

[0037] The upper sensor 10a is located on the thigh 12 and the lower sensor 10b is located on the calf 13. The purpose of the sensors is to monitor knee contraction at the knee joint, i.e., the pitch angle about the y-axis / knee joint axis 9. If the two sensors 10a, 10b can be aligned so that their z-axes are parallel to the respective mechanical axes of the leg and their y-axes are parallel to the knee joint axis 9, then the calculation of the knee angle becomes a simple subtraction of the calf pitch angle from the thigh pitch angle.

[0038] However, as will be appreciated, the shape and morphology of the human leg generally does not allow for such alignment, so when sensors 10a, 10b are in the position shown in FIG. 3, there is misalignment with the mechanical axes of the femur and tibia, which must be corrected in order to obtain an accurate measurement of the knee angle.

[0039] In embodiments where a patient has undergone a total knee replacement or, rather, any other knee surgery or knee disease that results in limited knee motion, it may be useful for a healthcare professional, or even the patient, to monitor the knee angle over an extended period of time, such as weeks or even months. This can present additional challenges due to the need to periodically remove the sensor for a number of reasons, including, but not limited to, cleaning the sensor, recharging the sensor, improving the patient's comfort at night, or cleaning the patient at the sensor's location. When a sensor is removed and reapplied, although this is done carefully, it is likely that there will be some misalignment of the replaced sensor relative to its previous position. When this occurs, the absolute value of the pitch or roll angle after replacement will not necessarily correlate to the absolute value of the pitch or roll angle before removal. Therefore, a method and / or device that increases the accuracy of sensor replacement and / or allows some form of correction for any misplacement would be beneficial.

[0040] 4 illustrates a sensor mounting system that allows one of the sensors 10a, 10b to be attached to each portion of the leg 11 in a manner that improves the accuracy of sensor replacement after removal.

[0041] The system is divided into two main parts: a first mount 20 shown in FIGS. 4a and 4c and a second mount 30 shown in FIG. 4b. The first mount 20 is intended to be placed directly on the patient and is the more permanent part. By this, we mean that the first mount 20 is intended to remain in place for a longer period of time than the second mount, such as a week. The second mount 30 is used to couple the sensor to the first mount and is intended to be used for a shorter period of time, such as a day. This may allow the sensor to be removed and recharged overnight, for example, at night, when knee movement is minimal and / or sleep is more comfortable for the patient. The first and second parts can be removably coupled together to allow the sensor to be applied to the patient.

[0042] The first mount 20 is a patch, typically formed from a series of four layers, as shown in FIG. 4a. Other numbers of layers are possible. In FIG. 4a, the first, bottom layer 21 is the outermost layer of the patch and may be formed from a material such as MED 5062A, which may be a flexible, transparent, breathable polyethylene film with an acrylic adhesive. The transparency is beneficial because it allows for visualization of the location. The adhesive side of the outermost layer faces layer 22. Layer 22 may be a polyurethane film to provide strength and durability to the mount 20. The polyurethane film may be a color that provides high contrast with the patient's skin to aid in sensor alignment during reapplication. The third layer 23 is typically a film with adhesive on both sides, such as MED 2171H, and preferably includes an absorbent hydrocolloid adhesive that is designed to resist breakage when penetrated, provides a low profile, helps create optimal skin and wound healing conditions, has high fluid handling capabilities, and is breathable. The fourth layer 24 is a release layer designed to be removed so that the patch can be applied to a patient's skin. The fourth layer may preferably include a release tab 25 or other protruding feature to aid in removal of the fourth layer from the third layer.

[0043] Each of the first through third layers has aligned or at least overlapping cutout portions 26 that, when the release layers are removed, allow the first outermost layer 21 to be visible through the patient's skin to which the patch is attached. The purpose of the cutouts is to aid in aligning a replacement first patch 20 in substantially the same position as the original patch, as they allow markings to be made on the patient's skin such that the marking(s) are visible through the patch, as will be described below.

[0044] The notches 26 can be holes through each layer (in which case markings can be easily replenished by marking through the holes) or can be transparent sections within each layer. A combination of the two may be used. While the notches 26 are shown as elongated stadium-shaped, other shapes can be used. While two notches are illustrated in the drawings, any number can be used. The number and / or shape of the notch(es) should aid in aligning the replacement first patch 20 in substantially the same position as the original patch. As an example, a single notch 26 can be used if the notch is shaped to allow for a certain orientation to be determined. For example, if a correspondingly shaped mark is present on the patient's skin, a single irregular cross or triangle may be sufficient to determine not only the position but also the orientation of the first mount 20. The notch has one dimension significantly larger than the other to help provide a satisfactory tolerance for orientation within the plane of the layers.

[0045] As can be seen in FIG. 4c, the second layer 22 and the third layer 23 are typically smaller than the first and fourth layers, such that when the release layer 24 is removed, the first layer 21 can be sealed onto the patient's skin around the second and third layers, i.e., completely enveloping them.

[0046] The first mount can be substantially smooth, in which the thickness is significantly smaller than the other two dimensions. One or more of the various layers in the first mount 20 can include a waist portion 27, where the layer narrows in one of the two larger dimensions. The waist is typically located at a point where the mount may bend, and the reduced size of the waist helps to create this bending. Additionally, the presence of a waist helps facilitate movement to pick up the mount from a flat surface. The first mount can be elongated, in which one of the two longer dimensions is at least twice the length of the other.

[0047] The second mount 30, or patch, shown in FIG. 4b, is bidirectionally secured. By this, we mean that two items can be joined together either by using a single structure with two bonding surfaces, each facing one of the two objects to be joined, or by using a two-part structure, each part connected to one of the objects to be joined and having complementary features that cooperate to join the two parts together. In each case, the second mount provides fixation in two opposing directions because it must bond to both the first mount and the sensor. The second mount 30 is typically slightly smaller than the sensor's footprint, so that when used as described below, any adhesive is not exposed even if the double-sided patch is not very well aligned. This also means that the sensor has a free, unbonded edge to facilitate peeling from the leg.

[0048] In the embodiment of Figure 4b, the second mount comprises three layers. The primary central layer 31 is a double-sided adhesive layer with a pair of outer release layers 32, 33. The central layer 31 is preferably a double-sided, conformable polyester film, typically with a solvent-free acrylic adhesive on both sides. This central layer 31 can be permeable or transparent. The central layer 31 is preferably conformable, vapor-resistant, breathable, and heat-sealable. Each of the outer release layers 32, 33 can have a release tab 34 or other protruding feature to aid in removal of these layers from the central layer.

[0049] As will be explained later, adhesive properties on both sides of the second mount or patch are used to attach the sensor to the first patch, thereby allowing the sensor to be secured on the patient as shown in Figure 3.

[0050] The second mount may be substantially planar, in which the thickness is significantly smaller than the other two dimensions. One or more of the various layers of the second mount 30 may include a waist portion 37, where the layer narrows in one of the two larger dimensions. The waist portion 37 of the second mount may provide benefits similar to those provided with respect to the first mount. The second mount may be elongated, in which one of the two longer dimensions is at least twice the other.

[0051] A further feature of the second mount 30 is a removal tab 35. The removal tab 35 is provided on at least the central layer and protrudes outside of the layer but is substantially in the same plane as the layer. The tab is typically integral with the remainder of the central layer. One or both of the release layers 32, 33 also have a corresponding tab. The central layer tab 35 is provided with a cover portion 38. The cover portion is intended to maintain the adhesive cover on the central layer when the release tabs 32, 33 are removed, thereby allowing the tab 35 to be used to assist in removing the central layer from the sensor to which it is applied or from the first patch 20.

[0052] Alternatively, the second mount may be formed from a two-part structure such as a hook and loop fastener or a press-fit fastener such as a popper, in which one part is secured to the first mount, either integrally or with an adhesive, and another part is secured to the sensor, also integrally or with an adhesive. Cooperating features such as the hook and loop or press-fit fastener hold the two parts together, thereby attaching the sensor to the patient. Velcro® may be "directional," whereby the hooks of the hook and loop all face the same direction, causing the fastening system to grip and hold better in one direction than the other, or potentially only in one direction and not the other.

[0053] In a further alternative, a clip on the first mount or sensor, or a pocket on the first mount, can be utilized as the second mount.

[0054] In a further alternative, one or more magnets may be utilized as the second mount.

[0055] In any of the embodiments, the sensor and / or the first mount (i.e., the sensor or the first mount, or both) may include one or more protrusions or the like that cooperate with the other of the sensor and the first mount to assist in aligning the sensor on the first mount.

[0056] 5-7 illustrate the use of first and second mounts in accordance with the discussion regarding FIG. 4, and thus also illustrate how the accuracy of sensor placement can be improved. These figures also illustrate how any misplacement of the sensor can be corrected by recalibrating the sensor. The correction method may utilize first and second mounts, as described herein, or may be implemented without the specific mounts or placement methods described.

[0057] FIG. 5 illustrates a simplified version of the correction method, which will be more easily understood once a more detailed method is described with reference to FIGS.

[0058] FIGS. 6a-6c show how sensors 10a, 10b are attached to a patient's leg. The leg is set in a baseline position, as shown in FIG. 6a. This position is preferably easily repeatable, particularly without the use of measuring devices, since the patient must be able to repeat this position away from medical facilities, i.e., at home. Semi-permanent markings 51 are applied to the leg at the intended sensor location. This may be done while the leg is in the baseline position shown, or it may be done at an earlier stage. Preferably, some form of goniometer 50 is used to record the knee angle in the baseline position. The goniometer preferably includes one or more templates 52 of the first mount's notches so that the semi-permanent markings match the notches. After marking, in FIG. 6b, the first mount or patch 20 can be applied by removing the release layer, and then the first mount is applied to the patient by aligning the notches 26 with the markings 51.

[0059] A second mount can then be used, typically attached first to the sensor and then to the first mount (see FIG. 6c). The shape and / or coloring of parts of the first mount can aid in aligning the sensor on the first mount.

[0060] The patient then returns their leg to a baseline position that can be checked, if necessary, by a goniometer and recorded knee angle ( FIG. 6 e) by entering the data into a mobile device 55, such as a phone. However, the sensor in this first position / orientation will necessarily be misaligned with the mechanical axes (femur and tibia), which must be corrected. This correction can be done by calibrating this first position to allow a pitch offset to be calculated / recorded, and then applying this offset to adjust the difference between the pitch reading from the sensor to the knee angle (pitch) previously measured by a healthcare professional, typically using a goniometer (or other suitable device). The first pitch offset is therefore the difference between the goniometer reading (knee angle) and the sensor reading. This allows the angle reported to the patient or healthcare professional to be determined in the knee's subsequent motion. This is because the reported angle is the sensor reading (which is variable) plus an offset (which is fixed here). Any of the data, including pitch / roll (i.e., pitch or roll) or orientation information, offset readings, and measured or reported knee angle, may be stored on one or more of the sensors and / or input into any form of computer-type device, such as a desktop computer, mobile phone, tablet, or laptop (i.e., stored on one or more of the sensors or input into any form of computer-type device, such as a desktop computer, mobile phone, tablet, or laptop). This input may be accomplished by automatically transmitting data from one of the sensors and may be synchronous, i.e., streamed for real-time use, or transmitted only periodically.

[0061] Typically, the first mount or patch will remain in place for a week before it needs to be removed to allow cleaning of the sensor location. However, over a shorter time period, such as the end of the day, the sensor may need to be removed along with the second mount (or parts of the second mount, if a two-part mount is used) for any of the reasons previously described. When replaced, the sensor 10a, 10b may or may not be replaced in the exact same position as before, so that the sensor has a second position. Therefore, before further useful readings can be taken, the patient must set their leg back to the baseline position, but without the aid of a goniometer or the like (because a position that is easy to repeat is preferred). The sensor must then be recalibrated in the same manner as above to provide a second pitch offset (the difference between the knee angle from the initial setup and the sensor reading taken in the second position). For knee motion after displacement, the reported angle is the sensor reading plus a second pitch offset. Preferably, the system for recording data regarding patient motion will not record new data until the offset has been updated.

[0062] Figure 7 shows how a first mount or patch can be replaced. First, in Figure 7a, the markings 51 on the legs 11 are replaced to ensure they are clearly visible. The first mount 20 is then removed (Figure 7b), allowing the area around the markings 51 to be cleaned and any hair removed (Figure 7c). A new first mount 20 can then be applied (Figure 7d), using the notch 26 and visual markings 51 on the first mount as a guide. Pressure can then be applied (Figure 7e) to ensure the first mount 20 is firmly secured in place.

[0063] Figure 5 describes the broad methodology associated with sensor compensation or calibration. In step 51, the joint to be monitored, and thus the joint around which two sensors are placed, one on either side of the joint, is set to a baseline position. This baseline position is the position shown in Figure 6d.

[0064] As mentioned above, it is useful to monitor movement after a total knee replacement, a situation in which a patient can typically straighten their leg but struggles to bend it. This makes sensors useful for tracking a patient's movement and potential improvement in movement over an extended period of time, such as weeks or months. For this reason, the preferred position is the "limit of movement position," and for the knee joint, this position is the passive full extension position. This position is effectively the position the leg would assume when extended along a horizontal surface.

[0065] Step 52 is to calibrate the sensors to a baseline position, whatever the angle of the knee. This calibration allows the sensors to set a first orientation (pitch and / or roll (i.e., pitch or roll, or both)) that they are set to correspond to the baseline position, so that any motion of the leg, and therefore the sensor, can be understood relative to that calibrated initial orientation.

[0066] As described, sensors can be removed for a number of reasons. While the method described with respect to FIGS. 6 and 7 helps reduce misalignment, it does not necessarily prevent misalignment from occurring, and so the sensor may be replaced with a different, second orientation. Any differences in orientation must be accounted for so that the data generated by the sensor after sensor replacement is similar to the data before replacement. For this reason, after a sensor is removed and replaced in step 53, the monitored joint must be set back to its baseline position, as in step 54. This may involve the use of a control system that only allows for recording and / or storing (i.e., recording and / or storing) further data after recalibration has occurred. The control system may be located on the sensor(s) themselves or may be located remotely from the sensors. The sensors may thus be recalibrated in step 55, thereby adjusting for any offset in the sensor's pitch and / or roll angle (i.e., pitch angle or roll angle, or both) relative to the initial reading. The initial reading of the baseline position may be updated, for example, by a healthcare professional, since the baseline position may change over time. This is especially true in the period immediately following surgery, when the patient is seeing a healthcare professional more regularly. Immediately after surgery, the patient may not be able to fully extend the knee, but after a week or several weeks, the patient may find that they are able to do so. Thus, the baseline position will change, and so the initial reading may need to be updated.

[0067] Applicant hereby discloses each individual feature described herein in isolation, and any combination of two or more such features, to the extent that such feature or combination can be implemented based on the specification as a whole in view of the common general knowledge of one of ordinary skill in the art, regardless of whether such feature or combination solves any problems disclosed herein, and without limiting the scope of the claims. Applicant indicates that aspects of the invention may consist of any such individual feature or combination of features. In view of the foregoing description, it will be apparent to one skilled in the art that various modifications may be made within the scope of the invention. The claims as originally filed are as follows: Claim 1: (a) determining a first offset between the angle between a pair of sensors, one mounted on each side of the joint to be measured, at a baseline position of the joint to be measured and the joint angle to be measured, thereby calibrating the sensors; (b) setting the joint back to the baseline position after at least one of the sensors has been removed and reattached, whereby the sensors are in a second configuration relative to one another; (c) determining a second offset between the measured knee angle and the angle between the pair of sensors in the second configuration to recalibrate the sensors, whereby the first configuration and the second configuration each report the same joint angle relative to the baseline position; 1. A method for calibrating a pair of body-worn sensors, comprising: Claim 2: The method of claim 1 , wherein the pair of sensors communicate such that the angle between the sensors is determined by one of the sensors. Claim 3: The method of claim 1 or claim 2, further comprising, before step (a), measuring the joint angle by using a goniometer. Claim 4: The method of claim 1 , wherein the recalibration is performed as part of a sensor start-up process. Claim 5: The method of claim 1 , wherein measuring the baseline position comprises measuring a joint angle between each part of the joint. Claim 6: The method of claim 5 , wherein the measured joint angle is a pitch angle or a roll angle or both. Claim 7: The method of any one of claims 1 to 6, further comprising the step of moving the joint to the baseline position, preferably in a fully extended position of the joint. Claim 8: 8. The method of claim 1, wherein the reattaching of the detached sensor is performed at substantially the same location. Claim 9: The method of claim 1 , further comprising identifying an axis of movement of the joint. Claim 10: The method of any one of claims 1 to 9, further comprising the step of attaching sensors, one on each side of the joint. Claim 11: The method of claim 1 , further comprising the step of marking a sensor location on each side of the joint before applying the sensors. Claim 12: a pair of sensors, each sensor being positioned on a respective side of the joint, in use, and each sensor including a data transmission device for providing data relating to the orientation of said sensor; a data storage device for receiving data from one or more of the sensors, the data relating to the orientation of one or both sensors; a control system configured to recognize when a sensor is removed from the joint and to require recalibration of the sensor alignment before recording a subsequent set of data; 1. A system for recording changes in angular position of a joint, comprising: Claim 13: 1. A system for attaching removable sensors to the body of an animal for a period of time, comprising: a first mount having an adhesive layer on one side for attachment to a surface of the animal's body during the first subset of periods; a second mount operative to removably secure the sensor to the first mount for a second subset of the period, the second subset being shorter than the first subset; and A system comprising: Claim 14: The system of claim 13 , wherein the second mount is two-way fixed. Claim 15: The second mount: (i) a first temporary fixation system for enabling the second mount to be fixed to the first mount during the second subset of the period; (ii) a second temporary fixation system for enabling a sensor to be fixed to the second mount; and 15. The system of claim 13 or 14, comprising: Claim 16: 16. The system of claim 15, further comprising a plurality of second mounts sufficient to allow repeated attachment of the sensor to the first mount during the first subset of periods. Claim 17: 16. The system of claim 13, further comprising a plurality of first mounts to enable replacement of the first mount after the first subset of the period. Claim 18: 18. The system of claim 13, wherein the second mount comprises one or more of an adhesive, a hard clip, a soft pocket, a press-fit fitting, a directional hook and loop fastener (Velcro®), or a magnet. Claim 19: 19. The system of claim 13, wherein the first mount includes at least one visual indicator section through which respective marks on the animal's body can be seen and which can assist in aligning a replacement first mount. Claim 20: 20. The system of claim 19, further comprising two or more visual indicator sections. Claim 21: 21. The system of any one of claims 13 to 20, wherein the first mount comprises a multi-layer structure, preferably having layers comprising MED 2171 H, polyurethane film, and MED 5062 A. Claim 22: 21. The system of claim 13, wherein the second mount has adhesive on both sides. Claim 23: 23. The system of claim 13, wherein the second mount includes a layer formed of MED 6361U. Claim 24: 19. The system of claim 13, wherein the second mount comprises two parts, a first part for attachment to the first mount and a second part for attachment to the sensor, such that when secured, the first and second parts are joined together. Claim 25: 25. The method of any one of claims 1 to 11, wherein one or more of the sensors are attached to the body using a system according to any one of claims 13 to 24.

Claims

1. (a) determining a first offset between a pitch angle and / or roll angle between a pair of sensors, one mounted on each side of the joint to be measured, at a baseline position of the joint to be measured, and the joint angle to be measured, for calibrating the sensors thereby, wherein the pair of sensors are in a first configuration relative to each other; (b) setting the joint back to the baseline position after at least one of the sensors has been removed and reattached, whereby the sensors are in a second configuration relative to one another; (c) determining a second offset between the measured joint angle and an angle that is a pitch angle or a roll angle, or both, between the pair of sensors in the second configuration to recalibrate the sensors, whereby the same joint angle relative to the baseline position is reported in each of the first and second configurations; 1. A method for calibrating a pair of body-worn sensors, comprising:

2. The method of claim 1 , wherein the pair of sensors communicate such that the angle between the sensors is determined by one of the sensors.

3. The method of claim 1 or claim 2, further comprising, before step (a), measuring the joint angle by using a goniometer.

4. The method of any one of claims 1 to 3, wherein the recalibration is performed as part of a sensor start-up process.

5. The method of any one of claims 1 to 4, wherein measuring the baseline position comprises measuring a joint angle between each part of the joint.

6. The method of claim 5 , wherein the measured joint angle is a pitch angle or a roll angle or both.

7. The method of any one of claims 1 to 6, further comprising the step of moving the joint to the baseline position, in which the joint is in a fully extended position.

8. The method of any one of claims 1 to 7, wherein reattaching the detached sensor is performed at substantially the same location.

9. The method of any one of claims 1 to 8, further comprising identifying an axis of movement of the joint.

10. The method of any preceding claim, further comprising applying sensors, one on each side of the joint.

11. The method of any one of claims 1 to 10, further comprising the step of marking a sensor location on each side of the joint prior to applying the sensors.

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